.gitlab-ci/check.yml: introduced check pipelines

.gitlab-ci.yml

 stages:
     - format
+    - check
 
 variables:
     GIT_STRATEGY: clone
 
 include:
   - local: /.gitlab-ci/format.yml
+  - local: /.gitlab-ci/check.yml

.gitlab-ci/check.yml

+.check-template:
+    stage: check
+    dependencies: []
+
+shellcheck:
+    extends:
+        - .check-template
+    script:
+        - .ci/check/shellcheck.sh
+    tags:
+        - ubuntu-24.04-amd64